This invention relates to a pierce nut and a back-up die used for securing the pierce nut to a metal sheet.
The pierce nut is known as a self-clinching pierce type fastener which is used as a punch to form an opening in a metal sheet through which the pierce nut is secured. When the pierce nut is secured to the metal sheet, it is used in combination with a back-up die.
Heretofore, several types of the pierce nuts and back-up dies have been developed as shown, for example, in U.S. Pat. No. 3,920,059. The fatal disadvantage of the pierce nut and back-up die combination of the prior art is that the life of the back-up die is very short. One of the reasons why the life of the back-up die of the prior art is very short is that the back-up die is subjected to heavy pressing force during the piercing operation of the nut, since press-breaking force is required for forming a full opening in the metal sheet for passage of the pierce nut. Another reason is that the load-bearing portions in the back-up die are always the same portions and therefore these portions are liable to be easily broken by heavy pressing force.
These reasons why the life of the prior art back-up die is very short will be further clearly explained with reference to FIGS. 1 and 2. FIGS. 1 and 2 show a pierce nut 1 and a back-up die 2 of the prior art which are used in combination with each other. The pierce nut 1 comprises a cylindrical body 3 and a radially extending flange 4 integrally formed with the cylindrical body 3. The cylindrical body has a center bore 5 in which a device such as a screw, bolt or pin is usually secured. It should be noted that no projections are formed on the outer peripheral surface of the cylindrical body 3 (FIG. 1). Instead, laterally inwardly extending projections 6 are formed in the back-up die (FIG. 2). A semi-spherical depression 7 is formed on the top of each projection 6.
When the pierce nut is punched into a metal sheet (not shown) laid on the back-up die 2, the cylindrical body 3 is inserted into a bore 9 of the back-up die 2 and the metal sheet is sheared between a bottom peripheral edge 8 of the pierce nut 1 and top peripheral edges 10 of the back-up die 2. However, the metal sheet is not sheared in portions at which the depressions 7 exist. Thus, these portions of the metal sheet have to be press-broken between a bottom surface 11 of the pierce nut 1 and the circular edges 12 of the depressions 7. Accordingly, the back-up die 2 of the prior art is subjected to heavy press-breaking force always at the same portions (circular edges 12) each time the back-up die is used and therefore, the circular edges 12 tend to be worn or sometimes even broken.